DESCRIPTION OF THE BACKGROUND ART
One of the most popular methods for obtaining high resolution images for television screen images is the HD-TV (high-definition television) method. On the other hand, as to a method for performing band compression in order to transmit video signals for this HD-TV, the MUSE(multiple sub-Nyquist sampling encoding) method has been developed. Moreover, nowadays several VTR's(video tape recorders) for recording these baseband signals and MUSE signals for HD-TV have already been suggested.
One of the VTR's suggested for recording baseband signals for HD-TV's is considered, for example, in Wideband Recording Technology for High-Definition Baseband VCRS, 1987 IEEE Vol. CE-33, No. 3).
In this VTR, in order to perform recording or reproducing of video signals having band limitations of 20 MHz for a brightness signal and 5 MHz for a color signal, the speed of rotation of the rotary drum is increased to 5400 rpm, which is three times that of conventional household VTR's. A segment recording method is also adopted, wherein one field is divided into three segments so as to be recorded in respective different tracks. Moreover, each of these divided tracks is divided into two parts for two-channel recording, thereby permitting a recording frequency to have a narrow band.
Compared with the above-mentioned baseband signal, a Muse signal can be compressed into about half of that size the above that so that a VTR is suggested, wherein the rotation speed of a rotary drum is twice that of conventional household VTR's. Furthermore, and one field is divided into two segments for performing one-channel recording. Accordingly, the VTR employing the MUSE signal is able to perform a longer recording than that employing the baseband signal.
However, some of these VTR's for recording baseband signals from HD-TV and the others for recording MUSE signals have been separately developed as different devices only aiming at use with the respective video signals. Conventionally, the VTR for recording baseband signals, which has a shorter a recording time, is therefore used in order to obtain high resolution images. On the other hand, the VTR used only for recording MUSE signals intended for satellite broadcast, which has a longer recording time. In this way, users have to select from among one of these VTR's depending on whether they want recorded images of high resolution or longer available recording time regardless of image quality. More specifically, they have been unable to perform long recording or reproducing with high resolution images in a single device. This problem is common wherein a selection can be made between video signals having a wide band and those having a narrow band.
Hereupon, the following description deals with a conventional video signal recording/reproducing device according to FIGS. 14 to 17.
As is shown in FIG. 14, there are four magnetic heads 22 secured to a rotary drum 21, forming two pairs. Each pair of magnetic heads 22 is located on the rotary drum 21 in an opposite position from each other with a 180.degree. interval. A video tape 23 supplied through a feeding side guide pole 24 and a winding side guide pole 25 is helically wrapped onto the rotary drum 21 at an angle of more than 180.degree..
The above-mentioned rotary drum 21 rotates at high speed, 3600 rpm, in order to increase relative speed between the magnetic heads 22 and the video tape 23. This is because baseband signals for HD-TV need a band of more than 20 MHz. Moreover, baseband signals having a wide band are recorded using a so-called two-channel, two-segment method by the four magnetic heads 22 located on the rotary drum 21, thereby permitting the magnetic heads 22 to have a narrow band as their recording band. More specifically, at first one field portion of a baseband signal is divided into two segments, and they are distributed to the respective pairs of magnetic heads 22 located at a 180.degree. interval. Next, each of the segments distributed is divided again into two channels, and simultaneously recorded respectively as two tracks by each pair of two magnetic heads 22.
Accordingly, as shown in FIG. 15, on the video tape 23, two tracks 26 corresponding to one segment are simultaneously formed every half a rotation of the rotary drum 21. Then, the total of four tracks 26 corresponding to two segments formed by one rotation of the rotary drum 21 compose one field, and the tracks 26 of two fields, each field including four tracks, compose one frame of baseband signal recording.
On the other hand, in a recording method wherein one field isn't divided into segments, the occurrence of noise bars on the screen can be prevented by performing the head switching of the magnetic heads in vertical blanking parts.
However, in the segment recording method, as described above, head switching among segments has to be performed not only in the vertical blanking parts but also in the screen.
Hereupon, the following description deals with a noise bar prevention processing method by using head switching with respect to conventional video signal recording/reproducing devices, according to FIG. 16. The left drawing in FIG. 16 shows one field portion of a baseband signal, and the right drawing shows a recorded signal of the baseband signal. These figures respectively represent one field portion of a baseband signal with its horizontal scanning period shown in the x-axis, and its vertical scanning period in the y-axis.
Besides an image part 27, in the one field portion of the baseband signal is contained a vertical blanking part 28 which has no relation to the images. Since the head switching between fields i performed in this vertical blanking part 28, conventionally there are no noise bars produced in the screen due to the head switching. However, since the head switching between segments has to be performed in a segment boarder 29 within the image part 27, as shown in the drawing on the left, there are noise bars produced within the above-mentioned image part 27 if no treatment is given.
In this relation, conventionally, the vertical blanking part 28 of the baseband signal is divided into two regions, vertical blanking parts 28a as well as 28b, as is shown in the drawing on the right. A signal wherein one of the vertical blanking part 28a is inserted into the segment boarder 29 located in the center of the image part 27 is used as a recording signal. In this case, by performing the head switching between segments in the vertical blanking part 28a inserted there into, the occurrence of noise bars in each segment of the image parts 27a and 27b is preventable. In this case, however, it is necessary to record the image part 27b with a delay of time T, and this process is performed in the circuit. In reproducing, it is necessary to compose the image parts 27a and 27b together by following the reverse operation.
As a result, in a normal reproduction, a correct image can be obtained on a screen 30 without noise bars, as is shown in FIG. 17(a).
However, in the segment recording method, in a special reproduction, the occurrence of divided images or the occurrence of noise bars on the screen 30 are not preventable. More specifically, when, for example, speed reproduction three times that of normal reproduction is executed in the segment recording method, an image on the screen 30 is divided into two upper and lower parts because magnetic heads 22 can not follow the one track. Moreover, in a special reproduction, the position of noise bars tends to move off the vertical blanking part 28a due to electrical malfunctions in the head switching synchronization because of the instability of the tape run or the like. As a result, as shown in FIG. 17(b), sometimes the image is not only divided into two parts on the screen 30 but also has noise bars 31 due to the head switching occurring in the boarder part between the two separated parts. This problem is common to a device employing the segment recording method, whether it is a video signal recording/reproducing device for HD-TV or not.
Accordingly, in the conventional video signal recording/reproducing device, there is a problem in that in a special reproduction, images are divided and also have noise bars near the boarders of the divided parts, and therefore the quality of those images deteriorates.
In this relation, for example, by using DTF(dynamic tracking following) technique wherein a piezo-electric device is employed to displace the magnetic head for the off-track portion, the scan can be performed following the one track and the division of images is preventable even in a special reproduction. However, the system is very complicated and delicate in mechanical structure as well as in control composition and its cost is very high, so in reality it is difficult to apply the DTF technique to video signal recording/reproducing devices for general use. Moreover, even if the DTF technique is adopted, the occurrence of noise bars due to the head switching between segments is not preventable, and therefore the deterioration of image quality is not completely avoidable.